Process of making cast iron of improved strength and machining properties

ABSTRACT

For making cast iron, a nucleating agent is added to the superheated melt consisting of highly dispersed silica or an alumina-silica or titaniumdioxide-silica mixed oxide containing at least 90 percent silica. A cast iron of improved strength and machining properties is obtained.

Inventor Heinz-Ulrich Doliwa Friedberg, Hessin, Germany Appl. No.808,301 Filed Mar. 18, 1969 Patented Nov. 2, 1971 Assignee DeutscheGold-Und Silber Scheideanstalt vormals Roessler Frankfurt am Main,Germany Priority Mar. 20, 1968 Germany P 17 58 004.0

PROCESS 01F MAKING CAST IRON OF IMPROVED STRENGTH AND MACHININGPROPERTIES 10 Claims, No Drawings Primary Examiner-L. Dewayne RutledgeAssistant Examiner-Joseph E. Legru Attorney-Michael S. Striker ABSTRACT:For making cast iron, a nucleating agent is added to the superheatedmelt consisting of highly dispersed silica or an alumina-silica ortitaniumdioxide-silica mixed oxide containing at least 90 percentsilica. A cast iron of improved strength and machining properties isobtained.

PROCESS OF MAKING CAST IRON OF IMPROVED STRENGTH AND MACHININGPROPERTIES BACKGROUND OF THE INVENTION The invention relates to aprocess of making cast iron of improved strength and machiningproperties, and more specifically to the use of specific nucleatingagents in such process.

The properties of cast iron are determined, on one hand, by the graphiteformation, and, on the other hand, by the structure of the metallicbase. Both items depend largely on the composition and on the melting,casting, and cooling conditions. in order to understand thecrystallization processes taking place in cast iron, it is important tobear in mind that the modifications of the iron-carbon phase diagramwhich are caused by the usual contents of cast iron in silicon andphosphorus cannot be disregarded as may be the case with carbon steel.It is, rather, necessary to rely on the corresponding three or morecomponent-phase diagram, in particular the iron-carbon-silicon diagramand the iron-carbon-phosphorus phase diagram. For the crystallization ofthe graphite from the melt, it is of great importance to determine thesubeutectic or supereutectic condition of the cast iron involved. Basedon a eutectic melt, the saturation degree S of the alloy can benumerically defined by the abridged formula used generally in industrialpractice, which reads as follows:

Si 4.23 3 In this formula, C is the carbon content and Si is the siliconcontact of the alloy expressed in percentages, while the number 4.23indicates the carbon content of the binary graphite eutecticum. With S,l, the cast iron is subeutectic, and with S, l it is supereutectic. Incase of graphite formation by way of laminae or flakes, the graphiteusually occurs in the form of more or less coarse irregularly curvedflakes which are frequently arranged in pockets. With subeutectic alloysit is no longer possible to discern a clear limit between the primarilyeliminated 'y-mixed crystals and the eutecticum in the solidifiedstructure. However, the primary precipitate of refined foam graphite, incase of a supereutectic composition, is clearly set off by itsparticularly coarse structure from the much finer structure of thegraphite of the eutecticum. The graphite eutecticum shows the finedistribution of the two components which is characteristic for aeutecticum only if an increased cooling rate and a correspondinglystronger supercooling is effected. The reason. for the tendency to formcoarse graphite flakes in the eutecticum is that the graphite as thedominating crystal structure in the eutecticum is strongly affected inits crystallization by nucleating forces.

lt is also well known that metallic additives that are soluble in themelt cause changes in the grain size in the direction of a finerstructure or also of a grain enlargement in the solidifying melt. On theother hand, foreign elements or metallic or nonmetallic compositionsthat are insoluble in the base metal show up in the final product as aseparate phase in the form of spheres, crystallites, or thin films whichare disposed at the grain boundaries or within the grains of the basemetal. The finely dispersed insoluble components in that case often actas stimulating nuclei and they add their action to spontaneous nucleiwhich may already be present and in general cause a finer grainstructure. Foreign nuclei useful for inoculation are both metallicprecipitates and nonmetallic occlusions, such as oxides, nitrides,sulfides, silicates, etc.

The composition of the cast iron has to be selected to secure agraphitic solidification under the cooling conditions employed as theyresult from wall thickness Cut the casting and the type of mold, such assand mold or ingot mold, cold or preheated mold, etc. This isaccomplished particularly by a predetermined selection of the siliconcontents which favors the graphitic solidification. An increase of thecarbon contents likewise acts in the same direction while, on the otherhand, a greater increase of manganese contents has the opposite effeet.Thus, with increasing cooling rate there is obtained an increasinglyfiner structure of the graphite formation.

The metallurgical pretreatment of the metal prior to casting likewisehas a substantial bearing on the graphite formation. The reason is thata superheating causes the crystallization nuclei to be put into solutionto a greater extent. This, in turn, causes more undercooling and thus afiner graphite structure.

A fine-grained solidification and fine dispersion of the graphiteprecipitation in the melt is therefore of greatest importance forhigh-grade cast iron types such as are required particularly for thethin-walled products which have come more and more into use in recentpractice.

Numerous processes which have the purpose to cause the cast iron meltsto solidify in a fine-grained structure and to cause a finely dispersedgraphite precipitation have been proposed on the basis of the work of E.Piwowarsky and his associates. These proposals are based on the additionof predetermined amounts of suitable nucleating agents aftersuperheating of the melt. This superheating and inoculation leads tohigh-strength cast iron types with excellent machining properties.

The inoculating agents used are predominantly FeSi, CaSi or othersilicon base mixed alloys. These agents can be added in the form ofloose granulates or in prepacked form. These agents have a deoxidizingeffect, partly also having desulfurizing action and furthermore improvethe eutectic structure of the cast iron. The amounts of additive are, ingeneral, for instance 4 kg. of CaSi per ton of iron.

However, the so far proposed additives for an inoculation treatment ofthe melt no longer meet the stringent requirements of modern castingtechniques, since the formation of a desired number of localcrystallization centers is limited by the lack of uniformity of theparticle size of the additives and by the differential amounts ofadditives used up for deoxidation and desulfurization. The obtainedgraphite laminae or flakes thus have different size dimensions.

It is therefore an object of the present invention to provide for a typeof nucleating agent in connection with the process described which willresult in a particularly high number of local crystallization centersand thus in a finely dispersed graphite. In a broader sense, it is anobject of the present invention to provide for an improvement in theprocess of making cast iron whereby a cast iron of better strength andbetter machining properties can be obtained when adding predeterminedamounts of nucleating agents which are no longer liquid when the meltsolidifies.

SUMMARY OF THE INVENTION These objects are accomplished by a processwherein a nucleating agent is added to the superimposed melt comprisinga highly dispersed silica or an alumina-silica or titaniumdioxide-silica mixed oxide containing at least percent of silica.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The highly dispersed silicaemployed in this invention may be an amorphous silica obtained in thegas phase by a pyrogenic process or may be a wet precipitated silica.The highly dispersed SiO because of its fine subdivision meets therequirement that the nucleating agent should result in the formation ofas many local crystallization centers as possible and should correspondto the lattice structure of the metal crystals. The inoculation withsuperfine SiO, results in a structure having a finely dispersed graphiteand in addition favors the graphite precipitation to an extent that theedge hardness which is caused by the preferential heat discharge at theedges of the casting is completely eliminated. It is this edge hardnesswhich causes subseque t difficulties in machining the metal.

A similar effect is obtainable by inoculation with mixed oxides.preferably containing from 90 to 98 percent SiO and from 2 to 10 percentA1 0 or TiO, The following is offered as an explanation for the effectsof the superfine amorphous silica or the mentioned mixed oxides withoutintention to restrict the inventors to this theory. It appears that thesuperfinely dispersed SiO, particles exert some kind of a directionalforce on the melt during cooling and that by wetting of the SiO:particles by the melt and by the presence of free surface forces, anincreased activity of the nucleating agent is provoked.

Regarding the making and properties of the pyrogenic process silica seeBritish Pats. No. 1,003,957 and No. 752,654 as well as the copendingapplication Ser. No. 189,236 filed Apr. 17, 1962. See furthermoreBritish Pats. No. 1,031,764 and No. 1,110,331 and copending applicationSer. No. 268,302, filed Mar. 27, 1963.

Regarding the precipitated silica reference is made to U. S. Pat. No.3,235,331.

It has been found that for accomplishing the effects of the invention,an addition of about to 300 g. preferably 100 to 200 g. SiO, per ton ofcast iron is adequate. That such small amounts of additives aresufficient is surprising. An explanation may be the fact that the highlydispersed silicic acids used as the nucleating agents are present duringthe solidification of the eutecticum in solid and not in liquid form. Afurther reason may be that, contrary to the mechanism in case of theaddition of CaSi or FeSi alloys, no uncontrolled formation ofdeoxidation or desulfurization products takes place.

The silica or mixed oxides may be used in the form of a prepackedcomposition, for instance it may be prepacked in a tin can. In order toassure a good distribution of the silica throughout the ladle, thepackage may be weighted down by materials such as FeMn, dry ironfilings, etc. The package will thus drop to the ground in the ladle. Apropelling agent may also be added, for instance in the form of anitrogen-generating mixture, in order to cause a whirling effect withconsequent thorough mixing and distribution ofthe silica.

The following example further illustrates the process of the invention.

The cast iron used in this example contained 3 percent carbon and 1.6percent silicon and had a saturation degree (S of 0.89.

EXAMPLE A highly dispersed silica in an amount of about 100 g./t. ofmelt was added to the melt in the form of a prepackage in a tin can. Thetin can in addition included weighting agents and a propelling agent.The weighting agents were dry iron filings, and the propelling agent wasa nitrogen-generating salt mixture.

This type of prepackage was thrown into the ladle, where it immediatelydropped to the bottom of the mold. The propelling agent had an intensewhirling effect and thus provided a uniform distribution of the silicathroughout the entire ladle.

What is claimed as new and desired to be protected by Letters Patent isset forth in the appended claims:

1. In a process of making cast iron of improved strength and machiningproperties wherein a nucleating agent is added to the preheated andsuperheated melt whereupon the melt solidifies, the use, as thenucleating agent, of a highly dispersed silica or an alumina-silica ortitanium dioxde-silica mixed oxide, the mixed oxide containing at leastpercent silica.

2. The process of claim 1, wherein the silica is a silica obtained inthe gas phase by a pyrogenic process.

3. The process of claim 1, wherein the silica is a wet precipitatedsilica.

4. The process of claim 1, wherein the size of the primary particles ofthe silica is between 5 and mm.

5. The process of claim 1, wherein the mixed oxide contains about 90 to18 percent Si0 and 2 to 10 percent Al,0 or "H0,-

6. The process ofclaim 1, wherein the silica or mixed oxide is added tothe melt in prepackaged form.

7. The process of claim 6, wherein the prepackage includes a wei htingagent. I

8. The process ofclaim 6, wherein the prepackage includes a propellingagent.

9. The process of claim 1, wherein the amount of silica or mixed oxideadded to the melt is from about 10 to 300 g./t. of cast iron.

10. The process of claim 1, wherein the amount of silica or mixed oxideadded to the melt is from about 100 to 200 g./t. of cast iron.

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2. The process of claim 1, wherein the silica is a silica obtained inthe gas phase by a pyrogenic process.
 3. The process of claim 1, whereinthe silica is a wet precipitated silica.
 4. The process of claim 1,wherein the size of the primary particles of the silica is between 5 and100 mm.
 5. The process of claim 1, wherein the mixed oxide containsabout 90 to 18 percent Si02 and 2 to 10 percent A1203 or Ti02.
 6. Theprocess of claim 1, wherein the silica or mixed oxide is added to themelt in prepackaged form.
 7. The process of claim 6, wherein theprepackage includes a weighting agent.
 8. The process of claim 6,wherein the prepackage includes a propelling agent.
 9. The process ofclaim 1, wherein the amount of silica or mixed oxide added to the meltis from about 10 to 300 g./t. of cast iron.
 10. The process of claim 1,wherein the amount of silica or mixed oxide added to the melt is fromabout 100 to 200 g./t. of cast iron.